Windows XP has power savings features built in, similar to the CPU-IDLE and CPU-COOL features known from the Windows 9X environment as part of the ACPI feature set. Cool'N Quiet was disabled in the BIOS to disallow change in core frequency.
A quote from my "software cooling" thread
When there is nothing to to, the operating system has the possibility to send three different signals to the CPU:
no operation, HLT, STPGNT.
Windows 9x versions without ACPI installed use "no operation".
Windows 2000 and XP without ACPI installed use HLT.
All Windows' with ACPI installed use ether STPGNT or HLT.
Linux without ACPI kernel support use HLT, with ACPI kernel support they use STPGNT.
There is a Linux-Howto available.
Once the chipset recieves a HLT or STPGNT is disconnects the FSB from the CPU. The CPU now uses a divider (8...512) to clock itself down significantly.
There is no powersaving difference between the HLT and the STPGNT command on an Athlon (XP) platform.
Enabling this feature doesn't bring any performance hit, as xbitlabs.com showed in this arcticle under "Heat Dissipation".
And from my Cool'n'Quiet thread
The Athlon 64 puts itself to sleep when the HLT/STPGNT instruction is sent.